Determination of iodothyronine absorption and conversion of L-thyroxine (T 4 ) to L-triiodothyronine (T 3 ) using turnover rate techniques

An assessment of daily production and significance of thyroidal secretion of 3, 3', 5'-triiodothyronine (reverse T3) in man

While 3, 3', 5'-triiodothyronine (reverse T3, rT3) has been detected both in human serum and in thyroglobulin, no quantitative assessment of its metabolic clearance rate (MCR), production rate (PR), or secretion by the thyroid is yet available. This study examines this information in euthyroid subjects and evaluates it in light of similar information about two other iodothyronines in the thyroid: 3, 5, 3'-triiodothyronine (T3) and thyroxine (T4). Thus, it was noted that rT3 is cleared from human serum at a much faster rate than are T3 and T4; the mean (+/-SE) MCR of rT3 was 76.7+/-5.4 liters/day in 10 subjects, whereas MCR-T3 and MCR-T4 in 8 of them were 26.0+/-2.2 liters/day and 1.02+/-0.06 liters/day, respectively. Therefore, even though the mean serum concentration of rT3, 48+/-2.8 ng/100 ml, was much lower than that (128+/-6.7 ng/100 ml) of T3, the mean PR-rT3 (36.5+/-2.8 mug/day) and the mean PR-T3 (33.5+/-3.7 mug/day) were similar; in comparison, the mean serum concentration and PR of T4 were 8.6+/-0.5 mug/100 ml and 87.0+/-3.9 mug/day, respecitvely. These data and those on the relative proportion of rT3, T3, and T4 in 10 thyroid glands were used to assess the significance of the contribution of thyroidal secretion to PR-rT3 and PR-T3. It was estimated that whereas thyroidal secretion may account for about 23.8% of serum T3 (or PR-T3), it may account for only about 2.5% of serum rT3 (or PR-rT3). Since peripheral metabolism of T4 is the only known source of rT3 and T3 other than the thyroidal secretion, it could be calculated that as much as 73.0 mug or 84% of daily PR-T4 may normally be metabolized by monodeiodination either to T3 or to rT3. MCR and PR of various iodothyronines were also examined in five cases with hepatic cirrhosis, where, as documented previously, serum rT3 may be elevated while serum T3 is diminished. The mean MCR-rT3 in these cases (41.0 liters/day) was clearly (P is less than 0.005) less than that (76.7 liters/day) in normal subjects. This was the case at a time when the mean MCR-T3 (26.7 liters/day) and the mean MCR-T4 (1.19 liters/day) did not differ from those (vide supra) in normal subjects. Distinct from changes in MCRs, the mean PR-rT3 (33.0 mug/day) was similar to, and the mean PR-T3 (10.1 mug/day) and the mean PR-T4 (66.4 mug/day) were much less than, the corresponding value in normal subjects. Furthermore, while the ratio of PR-rT3 and PR-T4 (rT3/T4) in individual patients was either supranormal or normal, the ratio of PR-T3 and PR-T4 (T3/T4) was clearly subnormal. The various data suggest that: (a) just as in the case of T3, the thyroid gland is a relatively minor source of rT3; peripheral metabolism of T4 is apparently its major source; (b) the bulk of T4 metabolized daily is monodeiodinated to T3 or to rT3; (c) monodeiodination may be an obligatory step in metabolism of T4; (d) monodeiodination of T4 to rT3 is maintained normal or is increased in hepatic cirrhosis at a time when monodeiodination of T4 to T3 is decreased.

Radioimmunoassay for 3,3',5'-triiodothyronine (reverse T3, R-T3) in unextracted human serum

Highly specific antibodies against 3,3',5'-triiodothyronine (reverse T3, R-T3) have been produced in rabbits. The crossreaction with T4 is about 0.05%. A radioimmunoassay for R-T3 in unextracted serum was developed. ANS is used for blocking the binding of tracer and endogenous R-T3 to TBG. The sensitivity to the assay is 0.06 ng/ml plasma. The mean normal R-T3 concentration is 0.20 ng/ml. Thyrotoxic patients show elevated levels; in most hypothyroid patients R-T3 concentrations are below the detection limit.

Relation of thyroid hormones to body-weight

Serum-triiodothyronine is positively and significantly correlated with body-weight. It is increased by overeating and reduced after weight-loss. No correlation has been found between serum-thyroxine and body-weight or changes in body-weight.

Serum triiodothyronine and reverse triiodothyronine concentrations after surgical operation

Serum-triiodothyronine (T3) concentrations fell rapidly after surgery in six out of seven initially euthyroid patients. Simultaneous increases in reverse triiodothyronine (rT3) concentrations suggested that the peripheral monodeiodination of thyroxine (T4) proceeds by an alternative pathway in the postoperative period.

Radioimmunoassay of 3,3',-'-triiodo-L-thyronine (reverse T3) in human serum and its application in different thyroid states

A radioimmunoassay for the measurement of 3.3',5'-triiodo-L-thyromine (reverse T3, rT3) has been developed. The known limitations of this technique have been overcome by the use of the biologically relevant L-compound for the production of highly specific antisera and for preparing the standard curve. The high sensitivity of the assay (lower limit of detection 20 ng/l serum) was obtained by using 125I-labelled rT3 of maximum specific radioactivity. Mean serum rT3 concentrations for various thyroid states were as follows: Normal subjects: 0.182 mug/l (0,280 nmol/l), hypothyroidism: 0.038 mug/l (0.058 nmol/l), hyperthyroidism: 0.522 mug/l (0.802 nmol/l), pregnants: 0.200 mug/l (0.307 nmol/l), newborn (cord serum): 2.11 mug/l (3.24 nmol/l). The method described should provide additional information with regard to the clarification of thyroxine metabolism.

Reduced peripheral conversion of thyroxine to triiodothyronine in patients with hepatic cirrhosis

The role of liver in the peripheral conversion of thyroxine (T4) to triiodothyronine (T3) was studied in normal subjects and patients with alcoholic liver disease by measurement of thyrotrophin (TSH) and total and free T4 and T3 in randomand serial serum samples. Also, T4 to T3 conversion rates and T3 disposal rates were compared by noncompartmental analysis. While the mean total serum T4 values were similar for the two groups, 8.6 and 8.1 mug/kl, the mean free T4 value was significantly higher in the cirrhotic patients (3.3 ng/dl) than in the normal subjects (2.1 ng/dl, P less than 0.001). The mean serum T3 value, 85 ng/dl, was significantly reduced in the hepatic patients as compared to a mean serum T3 value of 126 ng/dl in the normal subjects (P less than 0.001), while the free T3 value was 0.28 ng/dl in both groups. The reduction of the serum total and free T3 values were closely correlated with the degree of liver damage, as indicated by elevation of serum bilirubin (r equal -0.547) and reduction of serum albumin (r equal 0.471). The mean serum TSH level was 3.1 muU/ml in the normals and 7.1 muU/ml in the cirrhotic aptients ( less than 0.001). 15% of the hepatic patients had serum TSH values above 10 muU/ml, which, however, did not correlate with any of the four liver function tests studied. Serial blood sampling from two convalescing patients with alcoholic hepatitis showed a gradual normalization of serum TSH and T3 levels as the liver function improved. After oral T4 administration, 0.25 mg/day for 10 days, three of four cirrhotic patients studied failed to raise their serum T3 values. The mean T4 to T3 conversion rate of seven normal subjects was 35.7%. The mean T4 to T3 conversion rate of four cirrhotic patients studied was significantly reduced to 15.6% (P less than 0.001). The mean disposal rates of T4 and T3 of the normal subjects were 114 and 34 mug/day, respectively. The ratio of T4 disposal to T3 disposal was 3.5. In contrast, the mean T4 disposal rate, 82 mug/day, and the mean T3 disposal rate, 10 mug/day, were both reduced in the cirrhotic patients. Their ratio of T4 disposal to T3 disposal was 7.9. These findings suggest that impairment of T4 conversion in patients with advanced hepatic cirrhosis may lead to reduced T3 production and lowered serum T3 level. Therefore, the liver is one of the major sites of T4 conversion to T3.

Estimation of thyroxine and triiodothyronine distribution and of the conversion rate of thyroxine to triiodothyronine in man

Studies on peripheral metabolism of simultaneously administered 125-I-labeled L-thyroxine ([125-I]T4) and 131-I labeled L-trilodothyronine ([131-I]T3) were performed in five normal subjects, in four patients with untreated hypothyroidism, and in 3 hypothyroid patients made euthyroid by the administration of T4. The fractional turnover rate (lambda 03) of thyroid hormones irreversibly leaving the site of degradation and the volumes of pool 1 (serum V1) of pool (interstitial fluid, V2), and of pool 3 (all tissues, V3)were obtained by using a three-compartment analysis. In addition to the turnover studies, the ratios for the in vivo T4 to T3 conversion were determined by paper chromatographic study in sera obtained 4, 7, and 10 daysafter the injection. The rate (K12) of the extrathyroidal conversion of T4 to T3 was also estimated by the compartment analysis. The T3 distribution volume (V3) of pool 3, in which T3 is utilized and degraded, was about 60% of totaldistribution volume (V=V1+V2+V3) in normal subjects, whereas only about 25% of the extrathyroidal T4 pool was in the intracellular compartment, indicating that T3 is predominantly an intracellular hormone..

Reduction in extrathyroidal triiodothyronine production by propylthiouracil in man

To determine if propylthiouracil (PTU) inhibited extrathyroidal thyroxine (T4) to triiodothyronine (T3) conversion in man, PTU was administered to T4-treated hypothyroid patients and serial measurements of T4, T3, and thyrotropin (TSH) carried out. All patients had proven thyroidal hypothyroidism and had been receiving 0.1 or 0.2 mg T4 daily for at least 2 mo before study. Hormone measurements were made for 5 consecutive days before and daily during a 7-day treatment period with PTU, 1,000 mg/day. In eight patients receiving 0.1 mg T4 daily, administration of PTU resulted in a prompt fall in mean serum T3 concentrations from 78 plus or minus 6 ng/100 ml (SEM) to 61 plus or minus 3 ng/100 ml after 1 day. The mean serum T3 concentrations ranged from 55 to 60 ng/100 ml during the remainder of the PTU treatment period (P less than 0.01). The mean control serum TSH concentration was 29.6 muU/ml and it increased to a peak of 40 muU/ml on the 5th and 6th days. In five patients receiving 0.2 mg T4 daily, the mean control serum T3 concentration was 84 plus or minus 7 NG/100ML. It fell to 70 plus or minus 5 ng/100 ml after 1 day and 63 plus or minus 7 ng/100 ml after 2 days of PTU administration and thereafter ranged from 6) to 69 ng/100 ml (P LESS THAN 0.01). Serum TSH concentrations did not increase. No changes in serum T4 concentrations were found in either group. In five patients who received 100 mg methimazole (MMI) daily for 7 days there were no changes in serum T4, T3, or TSH concentrations. These results indicate that PTU, but not MMI, produces a prompt and sustained, albeit modest, reduction in serum T3 concentrations in patients whose sole or major source of T3 is ingested T4. These findings most likely result from inhibition of extrathyroidal formation of T3 from T4.

In vitro testing of thyroid function: a review

Triiodothyronine (T(3)) is the major thyroid hormone and thyroxine (T(4)) may be only a "prohormone". A normal serum T(3) concentration can compensate for a low serum T(4) to maintain euthyroidism and on the other hand hyperthyroidism can exist in spite of a normal T(4) if the T(3) concentration is increased ("T(3)-toxicosis"). A raised serum thyroid stimulating hormone (TSH) concentration is the present most sensitive indicator of thyroidal hypothyroidism and the level can be used to titrate replacement therapy to the individual's own requirements. TSH concentration is classically low in hypothyroidism secondary to pituitary or to hypothalamic disorder and synthetic thyrotrophin release hormone can then be used to identify which of these two sites is at fault. Thyroxine is the best form of thyroid replacement for hypothyroidism because it produces more consistently physiological concentrations of T(3). Full replacement is achieved with 0.1 - 0.2 mg of T(4)/day and doses above this, as formerly widely used, may cause over-replacement. New reliable kit tests are available which give in one quick procedure a measure of free-thyroxine even in the presence of abnormalities of protein-binding. These kit tests are suitable for the routine screening of the whole spectrum of thyroid dysfunction and when combined, in appropriate instances, with radioimmunoassay procedures for serum T(3) and TSH, provide a battery of tests which will help in the diagnosis of the great majority of causes of thyroid dysfunction.

A radioimmunoassay for measurement of 3,3',5'-triiodothyronine (reverse T3)

A highly specific antiserum to 3,3',5'-triiodothyronine (reverse T(3), rT(3)) was prepared by immunization of rabbits with D,L-rT(3)-human serum albumin conjugate. Of the various thyroid hormone derivatives tested, only 3,3'-diiodothyronine (3,3'-T(2)) cross-reacted significantly (10%) with rT(3)-binding sites on the antiserum, while thyroxine (T(4)) and triiodothyronine (T(3)) cross-reacted by less than 0.1%. The antiserum was used in a simple, sensitive, precise, and reproducible radioimmunoassay (RIA) for measurement of rT(3) in ethanolic extracts of serum. The dose-response curves of inhibition of the binding of [(125)I]rT(3) to antibody obtained by serial dilutions of serum extracts were essentially parallel to the standard assay curve. Recovery of nonradioactive rT(3) added to serum before extraction averaged 93%. Serum rT(3) concentrations were found to be (mean+/-SD) 41+/-10 ng/100 ml in 27 normal subjects, 103+/-49 ng/100 ml in 22 hyperthyroid patients, 19+/-9 ng/100 ml in 12 hypothyroid patients, and 54+/-7 ng/100 ml in five subjects with elevated serum thyroxine-binding globulin: the values in each of the latter three groups of individuals were significantly different from normal. Reverse T(3) was detected regularly in normal or supranormal concentrations in serum of 12 hypothyroid patients rendered euthyroid or mildly hyperthyroid by treatment with synthetic T(4). It is suggested that serum rT(3) values noted here should be taken to reflect the relative changes in serum rT(3) rather than its absolute values in health and thyroid disease. True serum rT(3) may be somewhat different because: (a) D.L-rT(3) employed in the standard curve and L-rT(3) present in human serum may react differently with anti-D,L-rT(2). (b) Even though 3,3'-T(2), which cross-reacted 10% in rT(3) RIA, has been considered unlikely to be present in human serum, it may circulate in low levels. (c) Cross-reaction of T(4) in rT(3) RIA of 0.06% although small, could contribute to RIA estimates of rT(2); the effect of T(4) would be particularly important in case of serum of hyperthyroid patients. Thus, serum rT(3) concentration in hyperthyroid patients averaged 89+/-48 mug/100 ml after correction for cross-reaction effects of T(4): this value was about 14% lower than that before correction (see above). Serum rT(3) concentration in cord sera of seven newborns averaged 136+/-19 ng/100 ml; it was clearly elevated and within the range of values seen in hyperthyroid patients. This was the case when the mean T(4) concentration in the newborn cord sera was moderately higher than normal and about one-half that in hyperthyroid patients, whereas serum T(3) was markedly below the normal adult level. A Pronase hydrolysate of thyroglobulin prepared from pooled normal thyroid glands contained 0.042, 3.0, and 0.16 mug/mg protein of rT(3), T(4), and T(3), respectively. The various data suggest that: (a) rT(3) is a normal component of human serum and thyroglobulin: (b) peripheral metabolism of T(4) is an important source of the rT(3) present in serum: (c) peripheral conversion of T(4) to T(3) and rT(3) may not necessarily be a random process.

Limited binding capacity sites for L-triiodothyronine in rat liver nuclei. Nuclear-cytoplasmic interrelation, binding constants, and cross-reactivity with L-thyroxine

Further studies have been performed to define the kinetic characteristics of nuclear triiodothyronine (T(3)) binding sites in rat liver (J. Clin. Endocrinol. Metab. 1972. 35: 330). Sequential determination of labeled T(3) associated with nuclei and cytoplasm over a 4-h period allowed analysis of the relationship of T(3) in nuclear and cytoplasmic compartments. A rapid interchange of hormone between nuclei and cytoplasm was demonstrated, and in vitro incubation experiments with nuclei yielded no evidence favoring metabolic transformation of T(3) by the nuclei. In vivo displacement experiments were performed by subcellular fractionation of liver (1/2) h after injection of [(125)I]T(3) with increasing quantities of unlabeled T(3). The nuclear binding capacity for T(3) could be defined (0.52 ng/mg DNA). Analysis of these experiments also allowed an estimation of the association constant of nuclear sites for T(3) (4.7 x 10(11)M(-1)). The affinity of these sites for T(3) was estimated to be 20-40 fold greater than for thyroxine (T(4)). Chromatographic analysis of the nuclear radioactivity after injection of labeled T(4) indicated that the binding of T(4) by the nucleus could not be attributed to in vivo conversion of T(4) to T(3) but reflected intrinsic cross-reactivity of the two iodothyronines at the nuclear binding sites.